Security documents such as banknotes now frequently carry optically variable devices such as diffraction gratings or holographic optical microstructures as a security feature against copy and counterfeit. This has been motivated by the progress in the fields of computer-based desktop publishing and scanning, which renders conventional security print technologies such as intaglio and offset printing more prone to attempts to replicate or mimic. Examples of such holographic structures and their manufacturing techniques can be found in EP0548142 and EP0632767 filed in the name of De La Rue Holographics Ltd.
The use of diffraction gratings or holographic optical microstructures has become more prevalent in recent years and consequently the underlying component technologies/sciences have become increasingly accessible to would be counterfeiters.
Optically variable devices can also be created using non-holographic micro-optics. One advantage is that mechanical copying of micro-optical components, such as microprisms, typically with a size range of 1-50 μm, is very difficult to achieve because any variation in dimension or geometrical distortion leads to a decline or extinction of the required optical properties.
The use of prismatic films to generate optical security devices is known. A grooved surface, a ruled array of tetrahedra, square pyramids or corner cube structures are examples of prismatic structures observed in such films. There is a significant volume of prior art on devices that utilise the retroreflective nature of prismatic structures. One example is EP1047960, which describes a reflective article with a concealed retroreflective pattern in which indicia are substantially hidden under normal viewing conditions but easily detectable under retroreflective lighting conditions. The general use of such devices is limited because in order to ensure correct verification of the hidden image the use of a directional light beam source is required which is typically in the form of handheld viewer.
An alternative application of prismatic structures in the field of optical security articles has been described in U.S. Pat. No. 5,591,527. In the preferred embodiment a substantially totally internal reflecting film, defined by a series of parallel linear prisms having planar facets, is adhered to a security document. A film comprising a plurality of parallel linear prisms can be used to produce an optically variable device using the phenomena of total internal reflection (TIR). A cross-section of a prismatic film defined by a series of parallel linear prisms is illustrated in FIG. 1. First consider the case where the film in FIG. 1 is viewed such that the light is incident upon the smooth surface i.e. the prismatic array is in a “prisms-down” configuration relative to the viewer. When the angle between facets is 90°, light incident upon the smooth surface at an angle θ1 to the normal of the smooth surface (ray 1) will be totally internally reflected at each face of the prism and exit back through the smooth surface when the incident light is refracted by the smooth surface and then strikes the facets of the structured surface (points a and b) at angles α1 and α2 respectively, with respect to the normal of the facet, which are greater than the critical angle. The critical angle for a material, in air, is defined as the arc sine of the reciprocal of the index of refraction of the material. In addition, a significant portion of the incident light striking the smooth surface at an angle θ2 to the normal of the smooth surface which produces refracted light that strikes the structured surface, for example at point c, at an angle, β1, less than the critical angle will be transmitted through the prismatic film (ray 2) and the remainder of the incident light will be reflected by the smooth surface. The switch angle, θspd, for the prisms-down configuration is the smallest angle of incidence with respect to the normal of the smooth surface at which the incident light is not totally internally reflected within the prism structure. The prismatic film in FIG. 1, when in the prisms-down configuration, exhibits an optical switch by being alternatively totally reflecting (bright “metallic” appearance) at angles of view less than the switch angle or transparent at angles of greater than the switch angle. In the totally reflecting state the film will exhibit a bright “metallic” appearance (i.e. exhibiting a lustre similar to that of metals), which is solely a result of the high reflectivity of the prismatic film. The film does not require a physical metallic layer, for example a vapour deposited metallised layer or a layer of metallic ink, to generate the bright metallic appearance.
In order to achieve TIR at the planar facet boundary in FIG. 1 the prism material must have a higher refractive index than the neighbouring material contacting the facets. U.S. Pat. No. 5,591,527 indicates that the change in refractive index at the planar facet boundary in FIG. 1 should be at least 0.1RI units and more preferably at least 0.7RI units. In the security article in U.S. Pat. No. 5,591,527 a significant refractive index difference is obtained by using a separation layer between the adhesive and the prismatic film to provide air pockets. In one embodiment the separation layer is provided in the form of an image in order to create a “flip-flop” image that is only viewable when the angle of view is greater than the critical angle.
Now consider the case where the film in FIG. 1 is viewed such that the light is incident upon the faceted surface i.e. the prismatic array is in a “prisms-up” configuration relative to the viewer. Light incident at an angle θ3 to the normal of the smooth surface (ray 3) is refracted by the faceted surface and then strikes the smooth boundary (point d) at an angle β2, with respect to the normal of the smooth boundary, which is less than the critical angle and therefore a significant portion of the incident light is transmitted through the prismatic film. In contrast light incident in a direction substantially parallel to the normal of the faceted surface (ray 4) at an angle θ4 to the smooth surface is refracted by the faceted surface and then strikes the smooth boundary (point e) at an angle α3, with respect to the normal of the smooth boundary, which is greater than the critical angle and therefore undergoes TIR and exits the prismatic film through the faceted surface at point f. The switch angle, θspu, for the prisms-up configuration is the smallest angle of incidence with respect to the normal of the smooth surface at which incident light is totally reflected by the prismatic structure. It should be noted that for the prisms-up configuration TIR only occurs for a limited angular range above θspu, and for angles of incidence exceeding this range the film switches back to being substantially transparent. This is discussed in more detail later in the specification with reference to FIG. 5. The prismatic film in FIG. 1, when in the prisms-up configuration, exhibits an optical switch by being substantially transparent at angles of view less than the switch angle and becoming totally reflecting (bright “metallic” appearance) at the switch angle and for a limited range above the switch angle and returning to a transparent appearance for angles of view exceeding this range.
A similar type of device to the one described in U.S. Pat. No. 5,591,527 is disclosed in patent applications WO03055692 and WO04062938. In this example a light-transmitting film with a high refractive index is applied to a product or document where one surface of the high refractive film has a prismatic structure. The film is placed over an image in the form of a legend, picture or pattern such that when viewed along the normal to the document the prismatic film is opaque and conceals the image but when viewed at an oblique angle the prismatic film is light transmitting allowing the image to be observed.
The security devices described in U.S. Pat. No. 5,591,527, WO03055692, and WO04062938 exhibit a distinct optical switch that is viewable in ambient light and therefore provides an advantage over the retroreflective devices that typically requires handheld viewers. However the devices described in the cited prior art contain only a simple on-off switch, i.e. the regions containing the prismatic structures switch from totally reflecting to transparent at the same specified angle, which limits the extent to which they can be customised. This limitation provides an advantage to the counterfeiter who only requires to produce one generic prismatic film that can be used to counterfeit a whole range of security devices. The current invention provides an optically variable security device based on a prismatic film where different regions of the prismatic film exhibit a different optically variable effect enabling the creation of a unique customised prismatic film for each security application.